1. Field of the Invention
The present invention relates to an inkjet printing apparatus and inkjet printing method. More particularly, the invention relates to an inkjet printing apparatus and inkjet printing method for suppressing error in amount of ink discharge and suppressing a decline in image quality.
2. Description of the Related Art
In an inkjet printing apparatus proposed heretofore, a plurality of printheads each having a plurality of printing elements are fixedly arranged in parallel and are caused to scan across a print medium to print on the medium. A characterizing feature of an inkjet printing apparatus having such a construction is a printing speed higher than that of a so-called serial-scanning-type printing apparatus for printing by the scanning of a printhead.
A problem that arises in the attainment of a high printing speed is a decline in image quality ascribable to a fluctuation in amount of ink discharge due to a temperature rise in the printhead. Various types of control for stabilizing the amount of ink discharged from a printhead have been proposed for the purpose of minimizing the occurrence of density unevenness, etc., in printed images and the like (see the specifications of Japanese Patent Application Laid-Open Nos. 5-31905 and 9-183222).
In an inkjet printing method available in the art, an ink bubbling force is produced by applying electric pulses to a heat-producing resistance element (also referred to as a “heater”), thereby heating the ink rapidly and causing the ink to undergo a change in state from the liquid phase to the gas phase. With this printing method, the amount of ink discharge is substantially decided by the method of introducing energy up to the change in state of the ink from the liquid phase to the gas phase. Consequently, after the ink has undergone the change in state to the gas phase, there is almost no effect upon the amount of ink discharge regardless of how the energy is introduced.
One conventional measure for dealing with a fluctuation in amount of ink discharge ascribable to a temperature rise in an inkjet printing apparatus is to control the method of energy introduction up to the change in state to the gas phase. For example, there is a method of modulating the amount of ink discharge by using divided pulses of the kind shown in FIG. 9 and controlling a preheating pulse, main heating pulse and quiescent time (interval time) between these pulses.
FIG. 9 is a time chart of heating pulses applied to a printhead. The heating pulses used here are divided pulses and the pulse width thereof can be modulated.
Pulse width and driving voltage Vop of the heating pulses for driving the printhead are decided by the area, resistance value and film structure of a heater board and the nozzle structure of the printhead.
In FIG. 9, reference characters P1, P2 and P3 denote a preheating pulse, interval time and a main heating pulse, respectively. The pulse waveform of at least one of P1, P2, and P3 is modulated based upon temperature information from a temperature sensor (a diode sensor, etc.) provided on the printhead. Further, reference characters T1, T2 and T3 represent the rise times of the applied pulses and indicate times for deciding P1, P2 and P3, respectively.
The preheating pulse P1 has a pulse width mainly for controlling ink temperature within a nozzle. This pulse width is controlled in accordance with temperature sensed utilizing the temperature sensor of the printhead. This pulse width is controlled in such a manner that the ink will not be caused to bubble by preheating owing to excessive application of thermal energy to the ink.
The interval time P2 is provided for the purpose of preventing mutual interference between the preheating pulse P1 and main heating pulse P3, and for the purpose of uniformalizing the temperature of the ink within the nozzle by causing the thermal energy applied by the preheating pulse P1 to spread into the ink at the portion above the heater.
The main heating pulse P3 subjects the ink to energy for bubbling the ink and discharging ink droplets from discharge ports.
In a case where a uniform image has been printed on the entire surface of the print medium, the temperature distribution along the row direction of the printing elements is not uniform, is high at the central portion of the row of printing elements and low at both ends thereof. In particular, at the end of printing when the temperature rise is great, this tendency becomes conspicuous, as indicated by the curve “ACTUAL TEMPERATURE DISTRIBUTION” in FIG. 2A. It should be noted that FIG. 2A is the temperature distribution of a row of printing elements at the end of printing in a case where a uniform image has been printed on the entire surface of the printing medium.
As a consequence of this non-uniform temperature distribution, the density of the image printed by the printing elements at the central portion of the row of printing elements exceeds the density of the image printed by the printing elements at both ends of the row, despite the fact that the intent was to print an image of uniform density. This invites a decline in image quality.
A conceivable method of controlling the amount of ink discharge in such cases is to hold the amount of ink discharge from each printing element substantially constant by selecting an optimum discharge pulse for each printing element in accordance with the temperature distribution along the row direction of the printing elements.
For example, in FIG. 2A, the amount of ink discharge is controlled using three types of discharge pulses with respect to the curve “TACTUAL TEMPERATURE DISTRIBUTION”. Discharge pulses from a Pulse Width Table No. 3 in FIG. 13 are used for the printing elements in areas A and E, in FIG. 2A. Since the printing elements in areas B and D have a higher temperature than that of the printing elements in areas A and E, discharge pulses from a Pulse Width Table No. 4 are used to suppress an increase in amount of ink discharge. Since printing elements in area C have an even higher temperature, discharge pulses from a Pulse Width Table No. 5 are used.
If the step-shaped line drawing labeled “DISCHARGE-PULSE SET TEMPERATURE DISTRIBUTION” in FIG. 2A is the temperature distribution along the row direction of the printing elements, the amount of discharge of ink droplets from the entire row of printing elements will be fixed, but the actual temperature distribution is the line drawing “ACTUAL TEMPERATURE DISTRIBUTION”. Accordingly, FIG. 2B is a diagram illustrating the temperature difference between the line drawing “ACTUAL TEMPERATURE DISTRIBUTION” and the line drawing “DISCHARGE-PULSE SET TEMPERATURE DISTRIBUTION”. FIG. 2B represents temperature error along the row of printing elements. According to FIG. 2B, the error has a width W2 centered on zero.
This error in amount of ink discharge can be reduced by increasing the types of discharge pulses used and changing the discharge pulses finely in accordance with the change in temperature. Accordingly, it will suffice to decide the number of types of discharge pulses in such a manner that the error in amount of discharge will fall within an allowable range. The allowable range of error in amount of ink discharge is decided depending upon whether a change in image density can be visually discerned, by way of example.
However, since a printed image is not always an image having a uniform density along the direction of the row of printing elements, the temperature distribution also is not necessarily as indicated by the curve “ACTUAL TEMPERATURE DISTRIBUTION” in FIG. 2A. For example, in a case where an image is printed using only some printing elements as in FIG. 7B, the temperature distribution of the printing elements in the direction of the row of printing elements becomes as indicated by line drawing A in FIG. 7A. Further, in a case where an image having a smaller width and a density that is lower than that of the image of FIG. 7B is printed as in FIG. 7C, the temperature distribution becomes as indicated by line drawing B in FIG. 7A. Although both curves indicate temperature distributions that are clearly different, the temperatures measured by temperature sensors at both ends are equal. When these two types of images originally having different temperature distributions are printed, how the discharge pulses are applied should differ as a matter of course. However, this cannot be distinguished because the temperatures measured by the temperature sensors are the same.
Thus, although various temperature distributions arise in actuality, it is difficult to predict these temperature distributions. For this reason, it has been contemplated to stabilize temperature by applying heating pulses having pulse widths in ranges that will not cause the discharge of ink to heaters other than heaters currently used in printing, in such a manner that a temperature difference will not arise among printing elements within the row of printing elements (see the specification of Japanese Patent Laid-Open No. 2001-239655).
FIG. 8 illustrates a heating pulse for performing heating (short-pulse heating) by a pulse within a range that will not produce a discharge of ink. The pulse width of a short pulse P4 is set to be shorter than a discharge pulse for performing ordinary printing.
A pulse within a range that will not produce a discharge of ink signifies a pulse that does not apply enough energy to cause ink to be discharged. A short pulse involves less consumed energy in comparison with a discharge pulse. In the case of a discharge pulse, however, heat is released by the ink droplet discharged. It is understood, therefore, that the energy that contributes to the head temperature rise from a pulse just small enough not to discharge ink is substantially equal to the energy that contributes to the head temperature rise from a discharge pulse.
Accordingly, if during printing the short pulse P4 is applied to heaters other than heaters used in printing (heaters to which a discharge pulse is applied), pulses equal to those of a fully uniform image can be applied for any image whatsoever. As a result, a temperature distribution similar to that of the curve “ACTUAL TEMPERATURE DISTRIBUTION” can be obtained at all times.
However, even in a case where the pulse applied is equal to that in a case where a uniform image has been printed over the entire surface of the printing medium, the temperature of the printing elements in the row direction of the printing elements is high for the printing elements at the central portion of the row of printing elements and low for the printing elements at both ends of the row, as mentioned above. In particular, at the end of printing when the temperature rise is great, this tendency becomes conspicuous, as indicated by the curve representing “ACTUAL TEMPERATURE DISTRIBUTION” in FIG. 2A.
As a result, even in a case where the same discharge pulse is applied to each printing element in order to perform the printing of an image having a uniform density, the density of the image printed by the printing elements at the central portion of the row of printing elements exceeds the density of the image printed by the printing elements at both ends of the row. Such a difference in density causes a decline in image quality.
In the prior art, as set forth above, there is no disclosure of a technique for preventing a fluctuation in amount of ink discharge satisfactorily, the fluctuation being ascribable to a rise in the temperature of the printing elements of the printhead.
In a case where an image having a uniform density is formed over the entire surface of the print medium, it is believed that the temperature distribution along the direction of the row of printing elements has left-right symmetry, as illustrated in FIG. 2A, and that the temperatures measured at both ends of the row of printing elements by two temperature sensors provided at both ends are equal. The reason for this is that the energy for heating the heaters is applied to each of the printing elements substantially uniformly. In actuality, however, it has been found that owing to uneven thickness, etc., of the bonding agent that bonds a printing element board and support board that construct the printhead, the temperature distribution becomes one that has left-right asymmetry, as illustrated in FIG. 3A, and hence there are instances where the temperatures measured by the two temperature sensors differ.
In this case, in a manner similar to that of FIG. 2A, three types of discharge pulses are decided based upon the average value of the temperatures measured by the two temperature sensors. When this is done, the line drawing labeled “DISCHARGE-PULSE SET TEMPERATURE DISTRIBUTION”; shifts greatly from the line drawing “ACTUAL TEMPERATURE DISTRIBUTION” especially near both ends of the row of printing elements, as illustrated in FIG. 3A. Consequently, the temperature difference between the line drawing “ACTUAL TEMPERATURE DISTRIBUTION” and the line drawing “DISCHARGE-PULSE SET TEMPERATURE DISTRIBUTION” increases, as illustrated in FIG. 3B. As illustrated in FIG. 3B, a value W3 indicating the width of the error is larger than the value W2 of the width of the error described in conjunction with FIG. 2B. The greater the temperature difference, i.e., the temperature error, the greater the fluctuation in amount of ink discharge. As a result, density unevenness becomes conspicuous and the quality of the image declines.
In particular, it is known that when use is made of a printhead having a plurality of printing element boards and the boards are placed in staggered fashion in such a manner that ends of the rows of printing elements slightly overlap each other, the decline in image quality becomes very noticeable. The reason for this is that owing to a fluctuation in amount of ink discharge at the ends of each row of printing elements, a difference in density at the portions of the image formed by the boundaries between the printing element boards becomes readily visually discernable and conspicuous.